Microstructure of Concrete affected by Delayed Ettringite Formation (DEF)
Delayed ettringite formation is a type of internal sulfate attack in concrete, which is common in many precast concrete elements that have been heat-treated beyond a certain temperature and have suppressed the normal ettringite formation, or, in a concrete that is made using a high-sulfate portland cement. In stead of normal formation of ettringite by cement hydration in the plastic state, in these concretes ettringite forms at a delayed state, after the hardening of the concrete. In the continued presence of moisture, components of ettringite (i.e. Ca, Al, S) slowly dissolves out and forms ettringite in the confined spaces in hardened paste, and thereby causes expansion, subsequent separations around the aggregate particles, stresses in the paste due to restrained expansion, and the resultant eventual cracking of concrete (similar to map cracking).
Following are some common microstructural evidences of DEF in concrete:
(1) Peripheral separations around the coarse aggregate particles due to bulk expansion of paste by DEF relative to the aggregate - the separations are highlighted by blue dye mixed epoxy impregnated polished concrete sections where the epoxy has preferentially penetrated along the separations at aggregate-paste interfaces
(2) Peripheral separations around the coarse aggregate particles due to bulk expansion of paste by DEF relative to the aggregate - the separations are
highlighted by blue dye mixed epoxy impregnated thin sections where the epoxy has preferentially penetrated along the separations at aggregate- paste interfaces
(3) Microcracking in the paste due to expansion by DEF
(4) Microcracks filled with ettringite fibers oriented perpendicular to the crack walls
